1. Field of the Invention
The present invention relates to an image bearing member protecting agent, a protective layer forming device, and an image forming apparatus.
2. Description of the Related Art
Conventionally, in electrophotographic image formation, a latent electrostatic image is formed on an image bearing member containing a photoconductive material, and charged toner particles are attached to the latent electrostatic image to thereby form a visible image. The visible image formed with the toner particles is finally transferred onto a recording medium such as paper, and then fixed on the recording medium under application of heat, pressure, a solvent, or gas to thereby form an output image.
Such electrophotographic image forming methods are broadly classified, depending on a method for charging toner particles to form a visible image, into so-called two-component developing methods in which toner particles and carrier particles are stirred and mixed so as to be frictionally charged; and so-called one-component developing methods in which toner particles are charged without using carrier particles.
Further, the one-component developing methods are classified into magnetic one-component developing methods and nonmagnetic one-component developing methods, depending on whether a magnetic force is used to keep toner particles held to a surface of a developing roller.
In copiers and complex machines based upon the copiers for which high-speed processing capability and image reproducibility are required, the two-component developing methods have been employed in many cases due to demands for stable chargeability of toner particles, stable charge rising properties of the toner particles, and long-term stability of image quality; whereas in compact printers and facsimiles for which space saving and cost reduction are required, the one-component developing methods have been employed in many cases.
Also, nowadays in particular, the technology of colorization of output images is progressing, and demands for improvement of image quality and stabilization of image quality are increasing like never before. For the improvement of image quality, toner particles have been made smaller in average particle diameter, and have been made rounder in shape with their angular parts removed.
Generally, in an image forming apparatus which operates in accordance with any such electrophotographic image forming method, regardless of which developing method is employed, a drum-shaped or belt-shaped image bearing member is uniformly charged while being rotated, a latent image pattern is formed on the image bearing member by, for example, laser light, and the latent image pattern is visualized as a toner image by a developing unit and transferred onto a recording medium. After the toner image has been transferred onto the recording medium, untransferred toner remains on the image bearing member. If such residual toner is directly conveyed to a region for the charging step, it may hinder the image bearing member from being uniformly charged. Accordingly, in general, the residual toner remaining on the image bearing member after the transfer step is removed at a cleaning step to thereby sufficiently clean the surface of the image bearing member, and then the charging is carried out.
Thus, various types of physical stress and electrical stress in each step in the image formation (e.g., a charging step, a developing step, a transfer step and a cleaning step) are applied onto the surface of the image bearing member, which changes a state thereof over a period of time. Of those stresses, frictional stress generated at the cleaning step causes abrasion of the image bearing member and scratches on its surface.
To solve this problem, that is, to reduce the friction between the image bearing member and a cleaning blade, there have been various proposals for image bearing member protecting agents and methods of supplying lubricant components and forming films (for example, see Japanese Patent Application Publication (JP-B) No. 51-22380, and Japanese Patent Application Laid-Open (JP-A) Nos. 2006-350240 and 2007-145993).
Recently, the present applicant has been proposed an image bearing member protecting agent which contains zinc stearate serving as a fatty acid metal salt and boron nitride serving as an inorganic lubricant for the purposes of preventing abrasion of an image bearing member, toner filming on the image bearing member, smear of a charging unit, and passing-through of toner (see JP-A Nos. 2009-282160 and 2009-300861).
In the above proposals, relatively large amount (i.e., 20% by mass) of boron nitride is incorporated. In this case, boron nitride having an aspect ratio of 15 to 35 (see Examples 3 to 5 in JP-A No. 2009-282160 and Examples 3 to 5 in JP-A No. 2009-300861) produces a satisfactory result such as excellent photoconductor-protecting property and less smear of a charging unit. However, it has been recognized that boron nitride having the aspect ratio of 50 or more reduces photoconductor-protecting property (see Comparative Examples 5 and 6 in JP-A No. 2009-282160 and Comparative Examples 6 to 11 in JP-A No. 2009-300861). Additionally, boron nitride is more expensive than zinc stearate (a price difference therebetween is 20 times or more), so that there has been a need for achieving good properties while the content of boron nitride serving as the inorganic lubricant is kept low.
Therefore, there is still room for consideration with respect to optimization of the aspect ratio of the inorganic lubricant, and adjustment of a mass ratio of the fatty acid metal salt to the inorganic lubricant (fatty acid metal salt/inorganic lubricant). Further, there has been a need for preventing abrasion of an image bearing member, toner filming on the image bearing member, smear of a charging unit, and passing-through of toner at a cleaning step while the content of the inorganic lubricant (in particular, boron nitride) is kept low.